Streams rich in aromatics including diolefins are often formed as by-products of hydrocarbon conversion processes. For example, Pyrolysis gasoline is often obtained as a by-product from thermal cracking of various hydrocarbons. The pyrolysis gasoline often includes many aromatic compounds, as well as diolefins (hydrocarbons with two sets of double bonds), mono-olefins (hydrocarbons with one double bond), alkanes with no double bonds, and sulfur and nitrogen compounds. Depending on the feed source to the thermal cracker, pyrolysis gasoline may also contain metal contaminants. Pyrolysis gasoline can be used as a source for aromatic compounds, but the diolefins, mono-olefins, sulfur and nitrogen compounds need to be removed before the aromatic compounds can be recovered by various processes, such as solvent extraction.
The pyrolysis gasoline is often treated in a two-step process prior to separating and purifying the aromatic compounds. This application addresses improvements to only the first step of the process. In the first step, diolefins and any alkynes are selectively hydrogenated to form mono-olefins and some paraffins. The first step is operated under moderate conditions with a selective catalyst such that primarily diolefins are reacted to mono-olefins. At the same time some of the mono-olefins are saturated and very few, if any, aromatic compounds are saturated. In the second step, additional mono-olefins are saturated (hydrogenated) to form alkanes, and the nitrogen and sulfur compounds are removed. The second step is operated under more severe reaction conditions, in the presence of a selective catalyst that would cause diolefins to polymerize and undesirably result in reactor pressure drop issues, therefore the first step is used to remove the more reactive diolefins prior to the second step.
The first step is operated at moderate conditions with a selective catalyst, so diolefins are reacted to mono-olefins, but relatively few mono-olefins are saturated and essentially no aromatic compounds saturated. The diolefins are far more reactive than the mono-olefins and aromatic species. The first step is often operated at a reactor inlet temperature of about 50 to about 150° C. with a delta temperature of up to about 20-50° C. across the reaction zone and a maximum outlet temperature of about 200 degrees centigrade (° C.) or less. The second step is often operated at an inlet temperature of about 250 to about 350° C. with about a 30-60° C. delta temperature across the reaction zone and a maximum outlet temperature of about 400° C. The diolefins and mono-olefins are hydrogenated in separate reactors, i.e. the first and second steps are conducted in separate reactors, to limit and control polymerization of the diolefins. Reducing mono-olefin hydrogenation reactions in the first stage limits excessive heat from the exothermic reaction that causes polymerization of diolefins. Over time deposit of heavy polymerate gradually accumulates and deactivates the catalyst, so periodically the catalyst needs to be hot hydrogen stripped or regenerated.
The claimed subject matter focuses on the first stage reactor section of the pyrolysis gasoline treatment process. In some examples, the configuration requires a large reactor effluent recycle stream, when high feed rates are used or if the diolefin concentration is high, or both. Hydrogen is often distributed to the first and second reactor beds in the first stage reaction section. The distributed hydrogen is used to more selectively saturate the diolefins and vinyl aromatics present in the feed. This has the potential to be hydrogen lean at the first reactor outlet and potential to create a higher temperature rise than desired if the reaction proceeds faster in the first bed than anticipated. If this occurs there is a possibility that the hydrogen lean environment can lead to polymerization type reactions which could result in plugging or shorter catalyst cycle.
Accordingly, it is desirable to develop methods and apparatuses for producing hydrocarbons. In addition, it is desirable to develop methods and apparatuses for reducing the reactor recycle rate by better utilizing the reactor effluent stream as a diluent for temperature control. Furthermore, other desirable features and characteristics of the methods and apparatus described herein will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.